Sennen Sensou Aigis: Shiro no Teikoku-hen

Many years after the goddess Aigis brought an end to the battle between humans and demons, the prince of Logres, descendant of the hero, was living a peaceful life. On one such day, his fiancée, the eastern princess Kaguya, came to him. But before they had time to worry about discussing marriage, demons appeared, and the royal capital was seized in the face of their might. The prince and his allies narrowly escape with Kaguya, and decide to retake the capital. (Source: Harem-Battle)

Long ago, the God Aigis sealed away the devils that caused havoc in the human realm. Now many thousands of years later the Demon God, Tiamat, has come to the human world seeking to destroy all humans using the very devils that Aigis had sealed away so long ago. One of the first to fall was the imperial capital where their prince, who is descended from heroes, had no choice but to run away and watch as his homeland burned. Now he must do what he can to survive Tiamat's onslaught and recruit allies in order to take back his kingdom. Will he live to see the day his kingdom is free of Tiamat's hands?! Source: My Anime List